Ctrl+F (Find) in Documentation

[irgolic]

In documentation (click a widget, press F1), there should be 'Find' functionality. Upon pressing Ctrl+F, a widget should pop up much like in other web browsers, taking text input and exposing a 'Next' and 'Previous' button, mapped to 'Enter' and 'Shift+Enter' shortcuts respectively.

QtWebEngine supposedly exposes some methods (QWebEngineView::findText) to help with this, but I think we have to implement the widget by ourselves. I'm sure someone has done this before, I suggest looking around and copying the skeleton of a widget from elsewhere if possible.